Firebird is an open source SQL relational database management system that runs on Linux, Windows, and a variety of Unix. The database forked from Borland's open source edition of InterBase in 2000, but since Firebird 1.5 the code has been largely rewritten .

Firebird inherited the sophisticated storage architecture of Interbase. For ensuring the ACID properties of transactions, the database engine keeps different versions of each record changed by the active users in the database. When the transactions are committed, the last version of every changed record is marked as the definitive. If transactions are rolled back, the database engine keeps the mark on the original record versions, leaving them untouched.

As a result, Firebird disk writes are very reduced in comparison with databases that use the traditional transaction log architecture. Transactions writing data don’t block another ones reading it and viceversa, because each one sees its own version of the database. The advantages of this design, however, have a tradeoff: some maintenance (“sweeping”) is required from time to time to clean-up old record versions and freeing disk space.